To fill a fuel reservoir with fuel, the usual practice is to insert a fuel pump nozzle into a filler pipe opening into the fuel reservoir, thereupon allowing fuel to be introduced into the fuel reservoir. To enable the fuel reservoir to be filled unhindered, the air in the fuel reservoir or the air/gas mixture in the fuel reservoir must be able to escape from the fuel tank since, otherwise, a pressure rise within the fuel tank would hinder the filling process.
To vent air from the fuel reservoir, one or more venting valves in fluid communication with a vent line are provided in the fuel reservoir, wherein the vent line can optionally be in fluid communication with an activated carbon filter. Air/gas mixture displaced from the fuel reservoir is either filtered by the activated carbon filter, ensuring that only small quantities of fuel vapor, if any, are released to the environment, or, in the absence of an activated carbon filter, are released directly to the atmosphere.
Venting and/or air admission valves known from the prior art, which are also referred to below simply as venting valves or shutoff valves, comprise a hollow valve housing, which has at least one communication opening, by means of which a valve housing interior is in fluid communication with the environment thereof. When the venting valve is installed in a fuel reservoir, the valve housing interior is in fluid communication via the communication opening with the fuel reservoir interior, thus allowing an exchange of fuel and of a fuel vapor/air mixture between the fuel reservoir interior and the valve housing interior via the communication opening. The valve housing interior is in fluid communication with a vent line via a ventilation opening arranged in a valve seat. A valve body, which can move freely in the valve housing interior and is also referred to as a float or float element or buoyant element, closes the ventilation opening at and above a predetermined operating fluid level within the fuel reservoir, thus preventing liquid and/or gas escaping from the valve housing. Below the predetermined operating fluid level, the valve body is at a distance from the ventilation opening, with the result that the valve housing interior and the ventilation line are in fluid communication.
The distance between the operating fluid level and the fuel reservoir inner wall on which the venting valve is fastened at which the valve body is subject to so much lift by the fuel that it closes the ventilation opening is referred to as the shutoff height.
If the fuel reservoir warms up, e.g. due to an adjacent exhaust system or due to a high ambient temperature, the vapor pressure of the fuel within the fuel reservoir rises. When the shutoff valve is closed by the fuel level and there is no second venting path or when the second venting path is closed by a pressure relief valve (also called a pressure retention valve), the tank internal pressure rises, causing the fuel reservoir to expand. This, in turn, has the result that the operating fluid level in the fuel reservoir falls, as a result of which too the valve body falls in the valve housing and moves away from the valve seat and thus exposes the ventilation opening. The excess pressure built up in the fuel reservoir can thus escape via the reopened valve. This process is repeated until the supply of heat has ceased or an equilibrium has established itself between the liberation of gas due to the supply of heat and leakage through the pressure relief valve.
It is desirable to be able to achieve different shutoff heights for an operating fluid reservoir, e.g. for a fuel reservoir. This may be necessary if a motor vehicle is to be sold in different countries, in which different regulations apply to fuel reservoirs, or if the same valve is to be used for several reservoirs. For this purpose, the prior art includes what are known as “riser pipe shutoff valves”, in which the lateral communication openings are extended as far down as the desired shutoff height in the installed position, making it possible to achieve closure of the venting valve or riser pipe shutoff valve in the case of a lower operating fluid level. For different shutoff heights, it is then necessary in each case to use different venting valves or riser pipe shutoff valves, each with different valve housings, in which the positions of the communication openings are matched to the desired shutoff height.
With this kind of riser pipe shutoff valve, it is possible during the filling of a fuel reservoir for the displaced fuel vapor/air mixture to be carried away via the communication opening and the vent opening to the activated carbon filter or to the atmosphere as long as the communication opening is not closed by the fuel in the fuel reservoir. With this type of venting valve, the shutoff height is the distance between the fuel reservoir inner wall and the upper edge of the communication opening. When the operating fluid level has reached the shutoff height, gas exchange between the fuel reservoir interior and the valve housing interior is no longer possible. If more fuel is introduced, it rises in the filler pipe, causing a corresponding rise in the pressure within the fuel reservoir, thereby raising the fuel column within the valve housing and hence also the valve body by virtue of the buoyancy thereof. Above a predetermined fuel level within the valve housing, the valve body closes the vent opening.
If a motor vehicle with a fuel reservoir having a correspondingly designed riser pipe shutoff valve is parked with a full or approximately full fuel tank and the fuel tank then warms up, the excess pressure which arises due to evaporation of the fuel should be discharged by pressure relief valves. However, the opening pressure of such a pressure relief valve is matched to the height of the filler pipe and, as a result, the fuel reservoir often expands before the pressure relief valve opens. In such a case, the operating fluid level in the fuel reservoir falls. As a result, an upper part of the communication opening is no longer closed by the fuel, allowing the fuel vapor/air mixture in the compensating volume of the fuel tank to spread through the upper section of the communication opening into the valve housing interior. Since the operating fluid level within the valve housing interior is higher than in the fuel reservoir interior, the fuel vapor/air mixture rises to the surface of the fuel column within the valve housing interior in the form of gas bubbles. This in turn reduces the lift on the valve body since the gas bubbles in the fuel reduce the effective density thereof. As a result, the valve body no longer closes the ventilation opening.
The gas bubbles rising to the surface of the fuel entrain fuel in the form of small droplets as they pass through the surface of the fuel, and these too spread out in the direction of the ventilation opening and pass through the ventilation opening into the ventilation line and, via the latter, into the activated carbon filter. If the motor vehicle is parked in a warm environment for a prolonged period, the process of fuel loss described above continues for a long period via the ventilation line, and significant quantities of fuel can thereby be lost via the venting and/or air admission system. As a result, the ability to function of the activated carbon filter is massively impaired, and more fuel vapor is released to the environment.